class Wacther{
    constructor(vm, key, cb){
        this.vm = vm
        this.cb = cb
        this.key = key
        Dep.target = this
        this.oldValue = vm[key]
        Dep.target = null
    }
    update(){
        const newValue = this.vm[this.key]
        if(this.oldValue === newValue){
            return
        }
        this.cb(newValue)
    }
}